In recent years, camera modules for taking photos have begun to be incorporated in mobile terminals such as mobile phones and lap-top computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be mass manufactured and marketed. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pickup devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modern image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, and excellent optical performance.
Compactness means in particular that a length from a lens edge of the lens system to an image pick-up surface should be as short as possible.
Low cost means in particular that the lens system should include as few lenses as possible; and that the lenses should be able to be formed from a resin or a plastic and be easily assembled.
Excellent optical performance can be classified into the following four main requirements:
First, a high brightness requirement, which means that the lens system should have a small F number (FNo.). Generally, the FNo. should be 2.8 or less.
Second, a wide angle requirement, which means that half of the field of view of the lens system should be 30° or more.
Third, a uniform illumination on the image surface requirement, which means that the lens system has few eclipses and/or narrows down an angle of incidence onto an image pick-up device.
Fourth, a high resolution requirement, which means that the lens system should appropriately correct fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration.
In a lens system which satisfies the low cost requirement, a single lens made from a resin or a plastic is desired. Typical such lens systems can be found in U.S. Pat. No. 6,297,915B1 and EP Pat. No. 1271215A2. However, even if the lens has two aspheric surfaces, it is difficult to achieve excellent optical performance, especially if a wide angle such as 70° is desired. Thus, the single lens system can generally only be used in a low-resolution image pickup device such as a CMOS. In addition, a thick lens is generally used for correcting aberrations. Thus, a ratio of a total length of the lens system to a focal length of the lens (L/f) is about 2. In other words, it is difficult to make the lens system compact.
In a lens system which satisfies the excellent optical performance requirement, three or even more lenses are desired. A typical three-lens system can be found in U.S. Pat. No. 5,940,219. A typical four-lens system can be found in U.S. Pat. Application Publication No. 2004/0012861. However, the addition of extra lenses increases costs proportionately.
In order to satisfy all the requirements of compactness, low cost and excellent optical performance, it is commonly believed that a two-lens system is desirable.
A well-known two-lens system is the retro-focus type lens system. A typical such lens system can be found in U.S. Pat. No. 6,449,105B1. The lens system comprises, from an object side to an image side, a first meniscus lens having negative refracting power and a convex surface on the object side, a stop, and a second meniscus lens having positive refracting power and a convex surface on the image side. The lens system helps correct aberrations related to wide angle of field of view. However, a shutter is positioned between the second lens and the image side, which adds to the distance between the second lens and the image side. Thus, the compactness of the lens system is limited.
In order to overcome the above described problems, an image pick-up lens system generally employs two lenses which are made from plastic or resin and which have four aspheric surfaces. A typical configuration of such system can be found in U.S. Pat. Application Publication No. 2004/0036983 and EP Pat. No. 1357414A1. The system can satisfy the compactness and low cost requirements. In addition, the system can provide a resolution of 300,000 pixels.
However, in U.S. Pat. Application Publication No. 2004/0036983, a ratio of a total length of such system to a total focal length of the lenses (L/f) is generally about 2. In addition, for correcting chromatic aberration, the Abbe constant ν1 of a first lens of the system must satisfy the condition ν1>50, and the Abbe constant ν2 of a second lens of the system must satisfy the condition ν2<40. In EP Pat. No. 1357414A1, a ratio of a total length of such system to a total focal length of the lenses is about 1.7; and such system must satisfy the condition ν1−ν2>25, wherein ν1 is the Abbe constant of a first lens of the system and ν2 is the Abbe constant of a second lens of the system. That is, in the two above-described systems, the smallest ratio obtainable is 1.7, which still constitutes a limitation on the compactness of the lens system. In addition, the two lenses must be made from different materials in order to correct chromatic aberration.
In one aspect, for perfectly correct chromatic aberration and to further improve optical performance, it is desirable that at least one lens of the system is made of glass. Glass is more expensive than other materials such as plastics or resins. Thus manufacturing costs are increased. In another aspect, molding is necessary for mass manufacturing of aspheric surfaces of the lenses. The different materials that need to be molded require different technical parameters to be applied during the molding process. This complicates the molding processes, and increases manufacturing costs. Further, plastic and resin materials are prone to absorb water. For example, the water absorbency of polymethyl methacrylate (PMMA) is 1.5%, and the water absorbency value of polycarbonate (PC) is 0.4%. Among the plastic or resin materials which can be suitably used to make lenses, only zeonex materials (polyolefin resins or cyclo-olefin polymers) have relatively low water absorbency. These water absorbencies are less than 0.01%. Zeonex materials are available from the Japanese Zeon Corporation. Therefore unless the non-glass lens is made from a zeonex material, it is liable to absorb water and deform. As a result, the optical performance of the system is diminished.
On the other hand, a CMOS having a resolution of 300,000 pixels requires relatively low optical performance from an image pick-up lens system. There is a need for a relatively low optical performance lens system which can satisfy the requirements of a CMOS having a resolution of 300,000 pixels. Because the optical performance and resolution are relatively low, the cost of such lens system needs to be commensurately low.
In summary, a low cost image pick-up lens system which has a compact configuration and which can be used in products having 300,000 pixels resolution is desired.